This invention relates to the field of mud pulse telemetry such as found in well logging, in particular when used with measurement while drilling (MWD) devices. More particularly, this invention relates to a new and improved valve scheme disposed in the path of mud flow in a drill string to provide pressure waves or pulses.
Mud pulse telemetry systems for communication from a downhole location in a drill string to the surface are well known in the art. These pulses comprise either a standing pressure wave which is generated by an oscillating valve or a series of pressure pulses which are also generated by a valve or other devices causing a partial obstruction in the flow of mud downhole. This obstruction (whether oscillating or a pulse mode) generates a positive pressure wave which permeates up the drilling mud in the drill string. This pressure wave is then detected at the surface. Examples of such positive pressure pulse telemetry systems include U.S. Pat. Nos. 4,655,289; 4,531,579; 3,958,217; 3,770,006; 3,982,224; and 3,997,876. In general, each of these patents disclose systems in which the flow of drilling fluid through the drill string is periodically restricted to send positive pressure pulses up the column of the drilling fluid to indicate a downhole condition.
Another method of mud pulse telemetry which is also well known involves venting a portion of the drilling fluid so as to change the resistance pressure and thereby send a negative pulsa wave up the drilling fluid to the surface. Examples of such negative pressure pulse telemetry systems include U.S. Pat. Nos. 4,405,021 and 4,351,037. These systems periodically vent drilling fluid from the drill string interior to an annular space between the drill string and the well bore to send negative pressure pulses to the surface in a coded sequence corresponding to a sensed downhole condition. It will be appreciated that the above references to such prior art patents being merely for purposes of illustration and not a complete listing of relevant patents in this field.
The positive pressure pulse telemetry systems generally require large amounts of power to partially restrict the flow of mud down the drill string in order to generate positive pressure pulses. These valves are controlled by large complex mechanical systems having a solenoid or some type of downhole motor. The negative pressure pulse telemetry systems require complex venting schemes and also require a significant amount of power to open or close the vent thereby overcoming the significant force of the flow of the drilling fluid. Thus, a need exists for a mud pulse telemetry system wherein the electrical and mechanical power required to generate the pulses are reduced.